Field of the invention
The present invention relates to an extremely low viscosity matrix epoxy resin system, more particularly this invention relates to a single component, extremely low viscosity, pre-catalyzed, high functionality diepoxide and epoxy resin system capable of being refrigerated and stored for extended periods of time with little or no change in ensuing physical or mechanical properties. Additionally, the present invention relates to an extremely low viscosity, high functionality resin system for suffusing into fibrous pre-forms to fabricate high strength, complex composite structures.
The cured epoxy resin system of this invention has excellent cold, hot, wet and mechanical, cohesive and adhesive properties. Epoxy resins, such as the suffusion resin system of this invention, have application throughout many industries. Typical examples of such industries are automotive, electronic, research, defense, medical and aerospace. In such industries the advancement of the art, especially that of aerospace and spacecraft composite structures development, is an on going process. Subsequently, it is widely known that many spin-off applications for lightweight and strong composite structures exist. An example of such an application is the government's involvement in the development of lightweight crash resistant rapid transit systems, earthquake resistant structures and orbital space structures.
Epoxy resins are resinous materials known to be valuable as the principle component in matrix systems, such as those involving high strength fibrous reinforcements.
When used as a matrix epoxy resin system it is generally necessary to add a curing mechanism in order to obtain polymerization. When a room temperature reactive curing agent is added to the resin, polymerization usually takes place at room temperature without the addition of external heating. Curing agents that react at room temperature tend to increase the resin viscosity from the moment of compounding so that in a short while the resin becomes too viscous to permeate either woven fabrics or unidirectional filaments. As the viscosity increases, the complete exclusion of air below the maximum allowable void content of 2 percent is difficult to nearly impossible to obtain. Most often the resin begins to polymerize before it can be properly utilized. For example, resin drawn into a fibrous matrix by vacuum often becomes too viscous to permeate the small interstices between fibers. Thus, it has become apparent that a need exists for a novel extremely low viscosity resin system with extended work and shelf life stability, that is cost effective and that eliminates the need for constitution before use.
When latent curing agents are added to a state of the art matrix epoxy resin system, polymerization does not take place at room temperature. A completely cured resin composition must be obtained by heating. While induced heat reduces the matrix epoxy resin system viscosity, often the viscosity is such at the outset that trapped air, common to all pre-impregnated resin and fiber compositions is not readily removed except by utilizing special and expensive processing methods. Complete processing often requires a special clean room lay up and vacuum bagging followed by autoclave or oven cure.
Epoxy pre-impregnated fiberglass and carbon fiber are typical examples of materials used to fabricate aerospace composite structures. Existing technology requires that these materials be arranged in a predetermined pattern on a expensive mold whose form controls their shape and size. Further the epoxy lay-up often must be sealed in a vacuum bag and heat cured in an oven or autoclave using vacuum as the principle means of evacuating air from the pre-impregnated lay-up. Thus thick lay-ups and complex shapes often must be cured using an autoclave that involves heat, vacuum and pressure up to 300 psi.
Alternative methods are injection molding and compression molding, both of which require considerable capital investment and complex tooling while precluding the use of continuous fiber reinforcements. State of the art composite fabrication methods, in general, involve long learning curves, costly materials, expensive labor and processing equipment.
Another process commonly referred to as RTM (resin transfer molding) often involves complex tooling, manufactured using 5 axis CNC machining or EDM (electrical discharge machining). The manufacturing process involves the pressure injection of a liquid resin into a pre-form of fibers that are contained in a closed and sealed RTM split mold. Most often, the RTM process requires the use of heat to reduce the viscosity of the currently available resins that are capable of producing acceptable performance properties while utilizing RTM technology. Cure commonly takes place in an oven, autoclave or by induction heating. RTM and EDM tooling methods involve high technology engineering that is not conducive to either limited production quantities or cost effective manufacturing.
In addition to the above, there is yet a very important expeditious process known to the trade as VARTM (vacuum assist resin transfer molding). The VARTM process employs adequate, yet very low cost molds upon the surface of which raw fiber lay-ups or prepared fiber pre-forms are placed. The lay-up and the mold are then either placed in a vacuum bag or the lay-up is contained on the mold surface by a vacuum film that is attached to the surface of the tool in such a manner that all air, on a practical basis, is evacuated. At this point resin is allowed to suffuse into the fiber. When the fiber is completely impregnated, closing the resin inlet while allowing the vacuum to exert the required negative pressure on the now impregnated lay-up terminates the resin suffusion. Cure occurs when either heat or time, or both, advances polymerization to a selected end point. Initial polymerization is followed by, for example, a "free standing" oven post cure that provides aircraft quality mechanical properties. "Free standing", means, for example, that a composite structure, after suffusion and initial cure, may be subjected to a final cure (to develop maximum properties) without mechanical support.
Yet another process commonly known as "filament winding" may utilize the product of this invention either by established filament winding methods or by utilizing aforementioned suffusion methods on a dry pre-wound form.
The present invention addresses the inherent limitations of current technologies by providing a low viscosity epoxy resin system that readily saturates, for example, but not limited to, woven fiberglass and carbon fabrics, unidirectional fiberglass and carbon tapes by both a suffusion and wicking action. These unique features are not available with any other resin system, especially at the use temperature of 20 to 25.degree. C.
More specifically the present invention provides for a one component, pre-catalyzed resin system that requires no vacuum degassing, weighing or mixing of ingredients. Additionally, the present invention provides a resin that is transparent which allows for visual and foreign matter monitoring of resin during usage. The ingredients of this invention when combined, provide a low order of toxicity, and an extremely long work life.